Ian Potter

The Vasse-Wonnerup is a shallow intermittently-open system located near the town of Busselton, Western Australia and is listed under the Ramsar Convention. Despite its ecological importance, the Vasse-Wonnerup is highly modified and suffers from excess nutrients, low oxygen levels, which can lead to fish kills. A major component of the fish that die during these kills is the iconic recreational species Black Bream (Acanthopagrus butcheri). Because Black Bream are a solely estuarine species, i.e. individuals complete their life cycle within the estuary and do not leave, depleted populations of this species cannot be replenished from stocks in the marine environment or from other estuaries. Results from previous studies in the Vasse-Wonnerup demonstrated that, following a major fish kill in April 2013 there was no evidence of recruitment (an increase in juveniles following the birth of new fish) of Black Bream from that year. It was not known, however, whether this was due to the environment at the time of spawning not being conducive for survival or to a lack of sufficient numbers of brood stock (sexually mature fish) and thus whether recruitment would continue to fail in the future. It also became apparent that the good, thorough information required for management of this fish in the Vasse-Wonnerup was lacking. There was thus an urgent need to acquire information on the key biological characteristics of Black Bream and assess the health of this stock in the Vasse-Wonnerup.

The Strategic Research Fund for the Marine Environment funded a range of floral and faunal research projects to determine the characteristics of the marine communities in the Jurien Bay Marine Park (JBMP), which was gazetted in 2003. This project has determined the diversity, density and species compositions of the fishes that occupy reefs, seagrass, unvegetated sand and nearshore surf zone habitats in three different types of management zones in the JBMP. The zones were (1) General use zones, where all types of fishing are allowed. (2) Scientific reference zones, where recreational and commercial rock lobster fishing and selected shore-based fishing activities are allowed. (3) Sanctuary zones, where no boat-based fishing is permitted. Future management of the marine park requires a sound understanding of the relationships among the fish faunas, both within and among the main habitat types in each of the different management zones, and of the variability that occurs at different spatial scales. The vision provided by the management plan for the JBMP was: “In the year 2025, the marine flora and fauna, habitats and water quality of the Jurien Bay Marine Park will be in the same or better condition than in the year 2005. The area will support viable and ecologically sustainable fishing, aquaculture, recreation and nature-based tourism and the marine park will be considered an important asset by the local community” (Anon., 2005). Key performance indicators, described in the management plan, were designed to ensure that this vision is met. This requires data on how estimates of abundance of fishes varied according to the type of sampling method used during this study. The baseline values for different sampling methods can then be used, in the future, to assess whether the vision for the marine park has been achieved. This study used the following complementary sampling methods to survey fishes in the different habitats in the JBMP, i.e. underwater visual census (UVC) and baited remote underwater video stations (BRUVS) over reefs, BRUVS and trawling in seagrass and over unvegetated sand and seine netting in surf zones. The combined results from the different methods demonstrated that the fish fauna of the JBMP is diverse and comprises temperate, sub-tropical and tropical species. Variability in the fish faunas was detected both within and among the main habitat types and was related to the range of physical and biological characteristics present. The compositions of species sampled by each method differed. In particular, whereas the samples obtained using UVC, trawling and seine netting contained a range of trophic groups, those collected employing BRUVS were dominated by carnivorous species. These differences in ichthyofaunal composition within and among habitat types and between sampling methods must thus be taken into account when selecting the methods used both for long-term monitoring of fish communities and for providing the types of data necessary for assessing whether the marine park is achieving its objectives.

The Vasse-Wonnerup Estuary is a relatively small, shallow and nutrient-enriched system which undergoes pronounced changes in salinity. The estuary provides an important habitat and refuge area for birds and, as a result, has been awarded RAMSAR status. However, despite the recognized importance of this system it has become severely degraded through the input of nutrients. This study aims to (i) describe the spatial and temporal trends benthic invertebrate faunal composition within the system and (ii) compare this fauna to that found in other degraded estuarine environments such as the Peel-Harvey and Swan-Canning in both the 1980s and more recently (2000s).

A total of 14,200 fishes was caught in the lower and middle regions of the Leschenault Estuary using a 21.5m seine net in each season between winter 2008 and autumn 2010. This total was only 3% less than the 14,601 fishes caught using the same seine net at the same sites twice seasonally in 1994, i.e. with the same amount of fishing effort. The numbers of species recorded in 2008-10 (36) and 1994 (33) were also similar. The above absence of a marked difference in the abundance of fish is consistent with the similarity in the mean densities of fishes per sample in the two periods. However, the mean number of species per sample and measures of diversity were greater in the current than earlier period. The eight most abundant species in 2008-10 ranked among the 11 most abundant species in 1994. Furthermore, the five most abundant species in 1994, which collectively accounted for ~ 90% of the total number of fish caught in that period, ranked amongst the top six species in 2008-10, recognising, however, that they contributed less, i.e. ~ 69%, to the total number of fish in that later period. These five species were the Elongate Hardyhead Atherinosoma elongata, the Sandy Sprat Hyperlophus vittatus, the Yelloweye Mullet Aldrichetta forsteri, the Silverfish Leptatherina presbyteroides and the Southern Longfin Goby Favonigobius lateralis. The far greater contributions of the most abundant species in 1994 than 2008-10 reflects the extreme dominance of the Southern Longfin Goby and Sandy Sprat in the earlier period. These two species thus contributed nearly 70% to the total number of fishes caught in the earlier period, compared with only 35% by the two most abundant species, i.e. Elongate Hardyhead and Sandy Sprat, in the later period. This helps account for the diversity of the fish fauna being less in the earlier period.

Managers, scientists and fishers now have an understanding of the implications of the age and size compositions, growth and total mortality of Black Bream and Estuary Cobbler and the current status of the stocks of those species in south coast estuaries. In particular, our fisheries-independent data has shown that the abundance of Estuary Cobbler in Wilson Inlet, which contributes by far the most of any estuary, to the commercial catches of this species, has declined markedly over the last 20 years. The outcomes of this project will assist in the development of plans aimed at sustaining the commercial and/or recreational fisheries for Black Bream and Estuary Cobbler and maintaining the environments of estuaries on the south coast of Western Australia. Such management plans can now be based on sound fisheries-independent data on the biology and contemporary status of those two species and knowledge of their relationships with the environment. Furthermore, managers can now be confident that the closure of certain areas within estuaries is an effective tool to protect the stocks of Estuary Cobbler in those estuaries. In addition, the implications of hypersalinity for the stocks of Black Bream within estuaries are now well understood by fishery managers and local communities. A wide understanding by fishers and members of local communities of the significance and benefits of the study has been created through their strong engagement with the research team during the course of the study. In addition, through their involvement in the study, two honours and a PhD student have been trained in contemporary techniques in fisheries science and population and community ecology.

OBJECTIVES 1. Devise quantitative and readily usable approaches for classifying the local-scale nearshore habitats within a range of estuaries in south-western Australia and predicting the habitat to which any nearshore site in those systems should be assigned. 2. Determine statistically how the compositions of the fish and benthic invertebrate assemblages in selected south-western Australian estuaries are related to habitat type. 3. Formulate a readily usable and reliable method for predicting which fish and benthic invertebrate species are likely to be abundant at any particular nearshore site in one of the above estuaries.

This study provides the sound quantitative data that are required by managers for developing plans for conserving the stocks of the Western Blue Groper Achoerodus gouldii, the Blue Morwong (previously Queen Snapper) Nemadactylus valenciennesi and the Yellowtail Flathead (previously Bar-tailed Flathead) Platycephalus endrachtensis in south-western Australian waters. The first two species are commercially and recreationally important in coastal waters and the third is one of the most important angling species in the Swan River Estuary. All three species have been identified by managers as requiring detailed studies of their biology, and Blue Morwong and Yellowtail Flathead are among a small suite of species selected as indicator species for the status of fish populations in marine and estuarine waters, respectively, in south-western Australia. As juveniles, Western Blue Groper typically occupy reef areas in protected inshore waters along the coast and around neighbouring islands. As the individuals of this species increase in size, they move offshore to deeper and more exposed waters over reefs. Spawning occurs in the latter environment, between early winter and mid-spring. The maximum length and age we recorded for Western Blue Groper were 1162 mm and 70 years, respectively, the latter age being the greatest by far yet recorded for any species of wrasse. However, most of the growth of this species occurs in the first 20 years of life. The Western Blue Groper is shown to be a monandric protogynous hermaphrodite, namely all of its individuals begin life as females and, after maturing, many subsequently change sex to males. Females typically first become mature at about 650 mm and 15-20 years and typically change to males at lengths of about 800-850 mm and ages of about 35- 39 years. As sex change takes place over a narrower range in lengths (650 to 900 mm) than in ages (15 to 49 years), that change is apparently related more to size than age. The fact that sex change is typically accompanied by a change in body colour from green to blue can be used to determine the approximate size at which females change to males, without having to cut open the fish to determine whether it possesses ovaries or testes. Growth curves fitted to the lengths at age of individuals of each sex of this hermaphroditic species using a novel technique demonstrated that, with increasing age, the lengths of males became increasingly greater than those of females. Thus, at ages 15, 30 and 60 years, the “average” lengths of females were approximately 600, 670 and 680 mm, respectively, those of males were approximately 695, 895 and 975 mm, respectively. As the Western Blue Groper is very long-lived and maturity and particularly sex change occur late, it is potentially very susceptible to overfishing. Thus, because the mortality estimates and per recruit analyses indicate that, at present, this species is close to or fully exploited, fisheries managers will need to take a precautionary and watchful approach to managing and thus conserving the stocks of this species. As with Western Blue Groper, the Blue Morwong moves to deeper, offshore waters as it increases in size and then matures and spawns in those waters. Although Blue Morwong has a maximum length of close to 1 m and thus, like Western Blue Groper, is a moderately large fish species, it has a far shorter life span, namely 21 years compared with 70 years. While female Blue Morwong do not grow to as large a size as their males (max. lengths = 846 and 984 mm, respectively), the maximum age of both sexes was 21 years. From the growth curves, the average lengths attained by ages 3, 6 and 10 years were 435, 587 and 662 mm, respectively, for females, compared with 446, 633 and 752 mm, respectively, for males. Both sexes exhibited little growth after 10 years of age. Juveniles of Blue Morwong less than 400 mm in total length were found exclusively in shallow, coastal waters on the south coast, whereas their adults were abundant in offshore waters of both the south and lower west coasts. The lengths and ages at which females and males typically mature in offshore waters of the south coast were about 600-800 mm and about 7-9 years. In contrast, the vast majority of females caught in offshore waters of the lower west coast (where they were of a similar length and age range to those in offshore waters on the south coast) became mature at lengths of 400-600 mm and 3-4 years of age. The attainment of maturity by Blue Morwong at far lesser lengths and ages on the lower west coast than south coast suggests that the former coast provides better environmental conditions for gonadal maturation and spawning. Furthermore, the contrast between the almost total absence of the juveniles of Blue Morwong in nearshore waters on the lower west coast and their substantial numbers in comparable waters on the south coast indicates that the larvae of this species produced on the lower west coast are transported southwards to the south coast, where they become juveniles. As spawning occurs between mid-summer and late autumn, the larvae, which spend a protracted period in the plankton, would be exposed, on the lower west coast, to the influence of the southwards-flowing Leeuwin Current at the time when that current is strongest. Although Blue Morwong is caught by recreational line fishing and commercial gillnet fishing when they are as young as 3-4 years, they do not become fully vulnerable to these fisheries until they are about 9 years old. Consequently, the individuals of this species can potentially breed over a number of years before they become particularly prone to capture by fishers. Mortality estimates and per recruit analyses suggest that the Blue Morwong in south-western Australia is currently not overfished. A greater resilience to fishing by Blue Morwong than Western Blue Groper reflects, in part, its shorter lifespan, gonochorism (namely, it is not hermaphroditic) and early maturity. The Yellowtail Flathead spawns in the Swan River Estuary between late spring and early autumn and completes the whole of its life cycle in this system. Although its females attain a far larger length (615 mm) than its males (374 mm), this species, unlike some of its relatives, is not a protandrous hermaphrodite, namely, it does not change from male to female with increasing body size. As the maximum age of both sexes is eight years, the far greater length attained by females is largely related to the far faster growth of that sex. Females outnumbered males in each age class in which the sample size exceeded 25, with the overall sex ratio being 2.7 females: 1 male. As the minimum legal length for retention of Yellowtail Flathead is 300 mm, and relatively few males exceed this length, the recreational fishery which targets this species is largely based on its females. The estimates of mortality and results of the per recruit analyses provided no evidence that the Yellowtail Flathead is currently overfished. From a management point of view, it is advantageous that the current size limit for Yellowtail Flathead exceeds the average length at which its females (259 mm) attain maturity. Furthermore, this species appears to be resilient to capture and release. The biological data provided in this study will be very useful for the ongoing development of management policies for three important commercial and/or recreational species in south-western Australian waters and will alert managers to the need to monitor closely the status of Western Blue Groper.

Data on the fish communities and environmental conditions in three normally-closed estuaries (Stokes, Culham and Hamersley inlets) on the central south coast of Western Australia have been obtained seasonally for three years. The sampling regime and analyses were designed so that the data and their implications would be of value to both fisheries and environmental managers. Salinities in all three estuaries rose as a result of a combination of salt loading through land clearing, dry winters and high evaporation rates during summer. These increases were most marked in the Culham and Hamersley inlets, eventually resulting in the salinities in these two estuaries exceeding by several times that of sea water. Massive mortalities of Black Bream occurred in these two estuaries when salinities were approximately twice that of sea water, a finding that has been published in an international journal. The development of extremely high salinities was accompanied by a reduction in the number of species and density of fish in Culham and Hamersley inlets, with only a small species of hardyhead surviving when salinities reached levels equivalent to four times sea water. Dietary data emphasise that Black Bream is a highly opportunistic omnivore and thus able to withstand major changes in potential food types. Survival by Black Bream over several years was greatest in Stokes Inlet, the most environmentally stable estuary. Growth of Black Bream varied greatly among estuaries, which appeared to reflect differences in density rather than diet. The results emphasise that (1) the stocks of Black Bream can only be sustained permanently in the basins of estuaries if the quality of environmental conditions in those systems is maintained at an appropriate level and (2) upstream pools can act as refugia for Black Bream when extreme conditions exist downstream.